This invention relates to thermal protective coatings. It is particularly useful as applied to coatings which are applied to substrates to protect the substrate from extremely high intensity, high velocity hyperthermal conditions.
Various compositions are known which provide protection against fire and other thermal extremes. Presently, such compositions generally include a polymeric binder and form a char when exposed to fire or hyperthermal conditions. The char-forming compositions may operate by various modalities. Several char-forming, fire-resistive coatings are described in Deogon, U.S. Pat. No. 5,591,791. Briefly, such coatings include ablative coatings, which swell to less than twice their original thickness when exposed to fire or other thermal extremes, intumescent coatings such as those disclosed in Nielsen et al., U.S. Pat. No. 2,680,077, Kaplan, U.S. Pat. No. 3,284,216, or Ward et al., U.S. Pat. No. 4,529,467, which swell to produce a char more than five times the original thickness of the coating, and subliming char-forming coatings of the type disclosed in Feldman, U.S. Pat. No. 3,849,178, which undergo an endothermic phase change and expand two to five times their original thickness to form a continuous porosity matrix. The intumescent and subliming coatings are denoted “active” thermal protective coatings. The coatings are also sometimes applied to an intermediate structure which is then applied to the substrate as set out in Feldman, U.S. Pat. No. 4,493,945.
The time required for a given temperature rise across a predetermined thickness of the composition, under specified heat flux, environmental, and temperature conditions, is a measure of the composition's effectiveness in providing thermal protection to an underlying substrate.
Eventually, the char is consumed by physical erosion and by chemical processes, such as oxidation by oxygen in the air and by free radicals produced by the coating or otherwise in a fire environment, and protection is substantially reduced. Before the char is totally consumed, degradation of the char layer leaves it crumbled and without the necessary strength to sustain itself, causing it to fail by being blown off or simply falling off (spalling).
Various methods and structures have also been used or proposed for applying these thermal protective coating materials. The most frequent approach is to apply the materials directly to the substrate, without additional structure. For many applications, however, a reinforcing material, such as fiberglass fabric, graphite fabric, or a wire mesh, has been embedded in the coating material to strengthen the material and prevent it from cracking or falling off the substrate under conditions of flame or thermal extreme. Examples of this approach are found in Feldman, U.S. Pat. No. 3,022,190, Billing et al, U.S. Pat. No. 3,913,290, Kaplan, U.S. Pat. No. 3,915,777, and Billing et al, U.S. Pat. No. 4,069,075. Sometimes the materials are first applied to a reinforcing structure such as a flexible tape or flexible wire mesh, and the combined structure is applied to the substrate. Examples of this approach are found in Feldman, U.S. Pat. No. 3,022,190, Pedlow, U.S. Pat. No. 4,018,962, Peterson et al, U.S. Pat. No. 4,064,359, Castle, U.S. Pat. No. 4,276,332, and Fryer et al, U.S. Pat. No. 4,292,358. In these last-mentioned systems, the purpose of the reinforcing structure may be both to strengthen the resulting composite and to permit its application to a substrate without directly spraying, troweling or painting the uncured coating materials onto the substrate. In any of the foregoing methods and structures, multiple layers are frequently applied to the substrate to provide additional protection.
Presently known materials and methods, however, are not as efficient, in terms of length of protection for a given weight of protective material, as desirable. Efficiency is particularly important because in many applications weight or volume is critically limited. Moreover, heavily loading coating materials with fire retardants may seriously impair their physical characteristics and otherwise limit their suitability as coatings, for example by limiting their film-forming characteristics or their water-resisting characteristics.
Under certain extreme fire conditions, all of these known coating systems have required excessive thickness and weight to provide adequate protection. One of the environments in which such extreme fire conditions can occur is in the vicinity of a delivery pipe carrying flammable compressed gas or liquid, typically a hydrocarbon, from one location to another location. A rupture in the pipe or a failure of a valve or joint can result in a high-temperature, high heat flux, high-velocity flame, frequently termed a “jet fire.” If the difference in pressure across the rupture or opening is greater than about two-to-one, a choked flow condition is produced at the aperture, and a supersonic flow of gas is produced downstream of the aperture. The heat flux of these high velocity gases is on the order of about 300 to 320 kilowatts per square meter, and the temperature can typically be from 1,000° C. to 1,500° C. There have been standards produced which define a jet fire and delineate test procedures for assessing the effectiveness of protective coating systems. An important standard is identified as OTI 95 634 “Jet Fire Resistance Test Of Passive Fire Protection Materials” (Health and Safety Executive (UK), Offshore Technology Report, 1996). This document is incorporated herein by reference.
When exposed to the temperatures, heat fluxes, and aerodynamic shear forces of a jet fire, presently known coating systems erode and are quickly consumed or spall and fall off. Ablative coatings tend to produce dense chars having good physical and chemical resistance, but in standard jet fire tests they have been found to allow an underlying substrate to reach the critical temperature in a very short time. In the case of active coatings which swell when exposed to thermal extremes, the degradations are usually in the form of fissures which are formed in the char as a result of differential thermal stresses produced by the high thermal gradients and rapid erosion caused by shear forces.
To increase the strength of char layers during exposure to thermal extremes, and to limit spalling and fissures, fabrics have long been incorporated in the coating materials. As set out in Feldman et al., U.S. Pat. No. 5,622,774, fiberglass fabric provides an inexpensive, easy to install, reinforcement in many high temperature applications. Jet fires, however, raise the fabric to temperatures above the softening point of the glass (around 870° C.), and the fiberglass fabric has disintegrated. Other fabrics have therefore been required. Graphite cloth, as taught in the foregoing Feldman et al., U.S. Pat. No. 5,622,774, and in Castle et al., U.S. Pat. No. 5,580,648, Boyd et al., U.S. Pat. No. 5,433,991, and Kobayashi et al., U.S. Pat. No. 5,401,793, is one choice. The graphite cloth may be either substantially pure carbon or a precursor material, as is well-known in the art. Refractory materials, such as quartz (Refrasil) fabric, are also used. Metal mesh is inexpensive and widely used, but it is heavy and difficult to install. Even when reinforced with fabric or mesh, however, known protective systems are not very efficient in protecting against a jet fire and therefore require thick, heavy coatings to provide even limited protection.
The patents mentioned herein are all incorporated herein by reference.